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The builders configuration key is a list of objects giving configuration for the Builders.
For more information on the function of Builders in Buildbot, see the Concepts chapter.
The class definition for the builder configuration is in buildbot.config.
However there is a much simpler way to use it, so in the configuration file, its use looks like:

These arguments specify the worker or workers that will be used by this Builder.
All workers names must appear in the workers configuration parameter.
Each worker can accommodate multiple builders.
The workernames parameter can be a list of names, while workername can specify only one worker.

factory

This is a buildbot.process.factory.BuildFactory instance which controls how the build is performed by defining the steps in the build.
Full details appear in their own section, Build Factories.

Other optional keys may be set on each BuilderConfig:

builddir

Specifies the name of a subdirectory of the master’s basedir in which everything related to this builder will be stored.
This holds build status information.
If not set, this parameter defaults to the builder name, with some characters escaped.
Each builder must have a unique build directory.

workerbuilddir

Specifies the name of a subdirectory (under the worker’s configured base directory) in which everything related to this builder will be placed on the worker.
This is where checkouts, compiles, and tests are run.
If not set, defaults to builddir.
If a worker is connected to multiple builders that share the same workerbuilddir, make sure the worker is set to run one build at a time or ensure this is fine to run multiple builds from the same directory simultaneously.

tags

If provided, this is a list of strings that identifies tags for the builder.
Status clients can limit themselves to a subset of the available tags.
A common use for this is to add new builders to your setup (for a new module, or for a new worker) that do not work correctly yet and allow you to integrate them with the active builders.
You can tag these new builders with a test tag, make your main status clients ignore them, and have only private status clients pick them up.
As soon as they work, you can move them over to the active tag.

nextWorker

If provided, this is a function that controls which worker will be assigned future jobs.
The function is passed three arguments, the Builder object which is assigning a new job, a list of WorkerForBuilder objects and the BuildRequest.
The function should return one of the WorkerForBuilder objects, or None if none of the available workers should be used.
As an example, for each worker in the list, worker.worker will be a Worker object, and worker.worker.workername is the worker’s name.
The function can optionally return a Deferred, which should fire with the same results.

nextBuild

If provided, this is a function that controls which build request will be handled next.
The function is passed two arguments, the Builder object which is assigning a new job, and a list of BuildRequest objects of pending builds.
The function should return one of the BuildRequest objects, or None if none of the pending builds should be started.
This function can optionally return a Deferred which should fire with the same results.

canStartBuild

If provided, this is a function that can veto whether a particular worker should be used for a given build request.
The function is passed three arguments: the Builder, a Worker, and a BuildRequest.
The function should return True if the combination is acceptable, or False otherwise.
This function can optionally return a Deferred which should fire with the same results.

A list of Locks (instances of buildbot.locks.WorkerLock or buildbot.locks.MasterLock) that should be acquired before starting a Build from this Builder.
Alternatively this could be a renderable that returns this list depending on properties of related to a build that is just about to be created.
This lets you defer picking the locks to acquire until it is known which Worker a build would get assigned to.
The properties available to the renderable include all properties that are set to the build before its first step excluding the properties that come from the build itself and the builddir property that comes from worker.
The Locks will be released when the build is complete.
Note that this is a list of actual Lock instances, not names.
Also note that all Locks must have unique names.
See Interlocks.

env

A Builder may be given a dictionary of environment variables in this parameter.
The variables are used in ShellCommand steps in builds created by this builder.
The environment variables will override anything in the worker’s environment.
Variables passed directly to a ShellCommand will override variables of the same name passed to the Builder.

For example, if you have a pool of identical workers it is often easier to manage variables like PATH from Buildbot rather than manually editing it inside of the workers’ environment.

When more than one build request is available for a builder, Buildbot can “collapse” the requests into a single build.
This is desirable when build requests arrive more quickly than the available workers can satisfy them, but has the drawback that separate results for each build are not available.

Requests are only candidated for a merge if both requests have exactly the same codebases.

This behavior can be controlled globally, using the collapseRequests parameter, and on a per-Builder basis, using the collapseRequests argument to the Builder configuration.
If collapseRequests is given, it completely overrides the global configuration.

Possible values for both collapseRequests configurations are:

True

Requests will be collapsed if their sourcestamp are compatible (see below for definition of compatible).

The BuilderConfig parameter nextBuild can be use to prioritize build requests within a builder.
Note that this is orthogonal to Prioritizing Builders, which controls the order in which builders are called on to start their builds.
The details of writing such a function are in Build Priority Functions.

Dynamic Trigger is a method which allows to trigger the same builder, with different parameters.
This method is used by frameworks which store the build config along side the source code like Buildbot_travis.
The drawback of this method is that it is difficult to extract statistics for similar builds.
The standard dashboards are not working well due to the fact that all the builds are on the same builder.

In order to overcome those drawbacks, Buildbot has the concept of virtual builder.
If a build has the property virtual_builder_name, it will automatically attach to that builder instead of the original builder.
That created virtual builder is not attached to any master and is only used for better sorting in the UI and better statistics.
The original builder and worker configuration is still used for all other build behaviors.

The virtual builder metadata is configured with the following properties: